This invention relates to a scanning electron microscope for producing a two-dimensional image by scanning a sample with a converged electron beam and digitally processing the secondary particle signal generated.
In observing a sample having a large surface roughness, a scanning electron image is blurred and a clear image cannot be obtained in the case where the focal depth of the electron beam is smaller than the roughness of the sample. In this case, a clear image can be obtained by reducing the angle of aperture of the electron beam and increasing the focal depth. A smaller angle of aperture, however, would increase the diameter of the electron beam due to the diffraction effect of a diaphragm and deteriorate the image resolution. Thus, the focal depth and the diameter of the electron beam are in the tradeoff relation to each other. On the assumption that the resolution of human eyes is limited and therefore the diameter of the electron beam should not be more than the resolution of human eyes against the scanning electron image, JP-A-1-236563 proposes an electron microscope in which the diameter of the electron beam is set to at least the minimum angle of aperture of the electron beam not more than the resolution of human eyes in accordance with the magnification change of the scanning electron image thereby to attain the maximum focal depth against the set magnification for observation.
In the scanning electron microscope recently used, the secondary particle signal is exclusively subjected to analog-to-digital conversion and projected on a display or the like. In the process, the scanning electron image is configured of pixels having a finite display area. The resolution of the scanning electron image, therefore, is not more than the visual field area corresponding to one pixel, i.e. what is called a pixel size. The resolution of the scanning electron image digitally processed in this way is described in “Nuclear Instruments and Methods in Physics Research A519 280”. In this reference, the resolution of the digital scanning electron image is evaluated using the information passing capacity (IPC) method. As long as the pixel size is sufficiently small as compared with the diameter of the electron beam, the resolution is substantially equal to the diameter of the electron beam, whereas the resolution is deteriorated with the increase in pixel size. The resolution of the scanning electron image for the image magnification at which the diameter of the electron beam is sufficiently smaller than the pixel size is at most 1.7 Lp, where Lp is the pixel size.